Are you suggesting that character development for Kylo is only recognized when he permanently kills something? Because the way I understand it, you recognize character development by actions, not the extraneous circumstances surrounding them. Especially if they only reveal themselves later on down the line.
I mean, if it was revealed that the guy he killed in TFA was actually a clone of Han Solo’s cousin and his real father is still hanging out somewhere else, I guess you wouldn’t think that affects the scene in TFA or Kylo Ren’s journey at all(?)
I can admit that may be a little too convenient for him (as in he doesn’t have to deal with the results of his own actions). But yes, I would still see it as character development. This is because he had no reason to believe it wasn’t his father, and yet he still killed him anyway.
EDIT: Forgot to mention that there still would be some consequences, at least in that scenario you described. I can’t imagine they would ever have a normal relationship again after that.